There is mounting evidence incriminating certain RNA viruses, which have genomic and antigenic homologies with oncornaviruses of other mammals, in the genesis of human malignancy. Exploiting these antigenic homologies, highly sensitive and specific radioimmunoassays will be developed for the purpose of detection of oncornavirus expression in humans. A radioimmunoassay for MuLV gs-3 antigen will be developed because the gs-3 antigen appears to be an interspecific mammalian C-type virus antigen and is, therefore, the best candidate for a human C-type viral antigen. The assay will be validated in animals bearing tumors that produce C-type particles before being applied to human studies. A radioimmunoassay for B-type murine mammary tumor virus (MMTV) antigen will be developed, since both murine and human breast cancers are associated with evidence of B-type particle production and since the MMTV antigen appears to have sequence homologies, as determined by immunologic cross-reactivity, with the putative human B-type particle. The assay will be validated in mice bearing mammary carcinomas producing B-type particles before being applied to detection of human viral antigen. In order to satisfy the requirements for materials for immunization, reference standard, and I125-labeled tracer antigen, viral gs antigens will be carried to various degrees of purity. Murine leukemia virus will be isolated in large quantities from tissue culture and mouse mammary tumor virus will be isolated from the milk of RIII/Haag mice. The virus will be disrupted by conventional means and the gs antigen purified by ge1 filtration, ion exchange chromatography, and preparative electrophoretic methods. Antigen used for iodinated tracer will be homogeneous, as determined by SDS polyacrylamide gel electrophoresis. Preliminary structural characterization of the gs antigens will be accomplished by standard physicochemical techniques.